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Short Circuit Current on 161kv line

  1. Jul 13, 2017 #1
    I work at a electric arc furnace. We measure a lot of items on our furnace and found that some weeks we preform well and other we do not (kwh/ton). With everything else being equal i believe that the utility's available short circuit current is affecting this.

    The utility feeds our substation 161kv + or - 7%. This is fed into a 50MVA 161kv to 13.8kv transformer. This 13.8kv is then distributed to our 45 mva arc furnace transformer that goes from 13.8kv to 500-800V.

    My question is, is there a way to calculate the available short circuit current on a system with only knowing my transformers and the incoming voltage. The 161kv line jumps 2-3kv when we arc our furnace. I figure this fluctuation in the 161kv line while running would give some indication on how stable their line is.
     
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  3. Jul 13, 2017 #2

    russ_watters

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    Do you mean the 161kv line DROPS by 2-3kv when the furnace is on?

    I'm not sure that counts as a short circuit rating issue (because it isn't a short to ground), but rather normal sag caused by the heavy load. At 2%, that is well within the utility spec (5-10% generally) and may not even be coming from the utility.

    Is this causing an actual problem?
     
  4. Jul 13, 2017 #3
    Yes it drops from like 168kv to 166kv... then jumps back once we stop running.

    well some weeks we preform well kwh/ton, and some weeks we don't. I would like to be able to see if this is what is causing our performance differences.
     
  5. Jul 13, 2017 #4

    berkeman

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    Well, keep measuring the line voltage and logging it, and see if it correlates to your production performance.
     
  6. Jul 13, 2017 #5

    anorlunda

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    I used to consult for utilities supplying arc furnaces. An arc furnace is the worst imaginable type of load.
    Your plant is probably causing troubles on the grid for many kilometers around; especially low voltages and harmonics.

    In the case I'm thinking about, the solution was to add 100 MVAR of capacitors at the plant's bus. That cost millions of dollars and it made a good fight between the utility and the plant owner about who should pay.

    No you can't calculate the short circuit voltage drop without knowledge of the utility's grid configuration in your city. That used to be public information, but in the USA after 911, it is a confidential national security secret. The only thing you can do is to sit down with the utility's engineers and discuss the situation.
     
  7. Jul 13, 2017 #6

    I am putting in an SVC (static VAR compensator) along with 3 different harmonic filters. This should help solve some of our issues but i was trying to see if i could calculate the benefit into dollars so that i could help justify it. Luckily we are on our own line 5 miles from the coal plant, but our backup feed is half the state away and yes it does pose a huge problem to everyone when the utility switches us over.
     
  8. Jul 13, 2017 #7

    anorlunda

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    Good. You're doing the right thing. But I think you should ask the utility's engineers to do the calculations for you. They have the software tools and they have the data needed. It is likely that they will be happy to cooperate. Your VARS will benefit them too.

    Edit: There are so secondary considerations, and things to check, about a static VAR installation. Typically more than a plant owner knows about or cares about. The utility engineers, or perhaps a consulting engineer should do the study. A study doesn't need to be expensive.
     
  9. Jul 13, 2017 #8
    We had a study done before we took bids for the SVC and we presented this information to all 3 vendors. I have read a lot on SVC's and how much they help EAFs but i was trying to see if i could get some actual values from other facilities. I wanted to quantify like electrode use, re-bricking, and kwh/ton. I have read of improvements of up to 10% on melt times and 3-5% on electrode and re-bricking. All these numbers have come from the vendors suppling the SVC. I'd rather have them from other furnaces.

    Or my original goal, an estimate of what we could do, based on what we experience currently. If i increased our short circuit current, basically what the SVC will accomplish.
     
  10. Jul 13, 2017 #9
    I think i'll just start graphing the voltage and current on the 161 lines and see if i can see any trends with production values...
     
  11. Jul 13, 2017 #10

    jim hardy

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    What controls current through your furnace ? My father-in-law's old Fiberfrax plant used saturable reactors. Surely there's newer stuff now.


    Stand some midnight shifts with the furnace operators? My guess is your differences lie in technique.
     
  12. Jul 13, 2017 #11

    Its all controlled by a computer. So it should be almost 100 percent the same. Only difference is the type of scrap we use. It measures everything, predicts the temperature, adds carbon and runs the natural gas jets. We use different taps on the transformer to control the current and arc length. It steps though them with a pre programmed cycle.
     
    Last edited: Jul 13, 2017
  13. Jul 14, 2017 #12
    For that situation - I highly recommend installing a good recording digital power meter / analyzer. - I do not doubt that the the utilities configuration is changing and may be affecting your performance. Trust me the utility hates what you do to their grid and at times may be trying to isolate your feed in a certain way so not disrupt other customers. A different X/R ratio will affect the dynamic performance of the AF.

    I had a lot of success with a company call ElectroIndustries - they have a trigerable waveform capture meter, that will provide a LOT of valuable info about what is really going on. For example you can trigger on many different event types, and record all V and I waveforms for analysis or to show to the utility.
     
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